Long Baseline neutrino oscillation experiment,
from KEK to Kamioka (K2K)
Neutrino mass is one of the great mysteries of the current understanding
of elementary particle physics. To verify or deny the possibility of neutrino
mass, we are starting a long baseline neutrino oscillation experiment (K2K),
using the 12GeV proton synchrotron accelerator at the High Energy Accelerator
Research Organization (KEK) in Tsukuba city in Ibaraki Prefecture, and
the Super-Kamiokande detector which lies 250km away in Kamioka town in
Gifu Prefecture. At the KEK site, we create a neutrino beam and do a very
precise measurement of the beam flux and the coutamination of wrong species of neutrino, using a set of nearby detectors. Then we can look
for any possible effects due to neutrino oscillations by comparing the
Super-Kamiokande measurement with the front detector measurement.
The long baseline neutrino oscillation experiment, K2K:
The above piece of a map of Japan has some important points about the K2K
experiment in it. Basically, the thick black arrow is the 250 Km path which
the neutrinos will follow from KEK to Super-Kamiokande. The scenic northern
"Japan Alps" under which our beam will travel. The central expanded
section on the right is a caraciture of the KEK laboratory grounds, with
further details shown depicting our beam target area and front detector
At KEK, the intense proton beam from 12GeV proton synchrotron produces
numerous pi-mesons at the production target made of aluminum. These pi-mesons
are focused to decay into muons and neutrinos in the 200-m decay pipe.
The neutrinos emerged from the dump shield pass through the front detector
and travel to Super-Kamiokande within a millisecond.
We have a 1 kiloton water Cherenkov detector and a
fine grain scintillating fiber detector to measure
the neutrino flux and beam composition at the KEK site. Behind the fine
grain detector, there is an array of lead glass
to measure the beam composition, and then a "Muon
ranger" which is a stack of iron plates and wire chamber detectors
to measure the muon spectrum accurately. This whole suite of detectors
sits in a hole below ground level so that the beam
center can be properly aimed at the far detector. Because of the earth's
curvature the straight line from the target to Super-Kamiokande points
down by about one and a half degrees into the ground.
Front detector hall of K2K:
Some photos of these components at their present stage of deployment are
shown below. We are sorry that some are not quite current, but as you can
see, we are very busy working on the detctors.
Current Status of K2K (July, 1998)
Now our experiment (K2K) (like our web pages) is still under construction.
It is scheduled to start on January 27,1999. The neutrino beam line and
front detector construction are on schedule. The far detector (Super-Kamiokande)
has been operating to take cosmic ray data since April 1, 1996.
Here are some photos we have taken to document the construction progress.
Aerial view of the neutrino beam line ('98.02).
The bending section from the building at the bottom left is the end of
the proton beam line which now extends to the target station near the center
of this photo. The straight section in the upper middle of this picture
(pointing just to the left of up) is the decay tunnel for pions. Near the
top, just before the trees, one just can see the round hole where the front
detector hall is built. The kiloton water Cherenkov detector, fine grain
detector, lead glass array, and muon range stack will all operate in this
The magnetic horn is waiting to be installed.
After the proton beam strikes our aluminum target and makes pions, we focus
those pions with a pair of "horn magnets" (named for the shape
of the inside part) to make them travel straight along the decay tunnel.
By doing this, we are able to increase the number of neutrinos that reach
our far detector by a large factor. The horns are produced and have been
tested both mechanically and electrically. Soon, we will place them in
the target station at the end of the proton beam line.
The beam line magnets are being installed. ('98.07)
This is for the last section of proton transport before the target station
(-the curved section in the above photo).
The experimental hall ('98.05).
As of July, 1998, the iron plates for the muon ranger, the full lead glass
detector, and the tank for the 1 kiloton of water are all installed. (See
below!) In this view of the hall, the beam will come from approximately
the bottom left. The dark red (painted) structure in the photo is support
for the muon ranger and the lead glass detector.
The muon ranger's iron plates being installed ('98.06).
The iron plates for the muon range detector are now all in place, and have
holders (not in picture) for the wire chambers attached to them. By following
muon tracks through these plates, we can measure the spectrum of muons
produced by neutrino interactions very well.
The lead glass detector was installed in the experimental hall. ('98.06)
This view is almost along the beam direction. The array of lead glass detectors
will measure how many electrons are produced in the fine grain detector.
This is very important to know so that we can predict any background of
expected electron neutrino interactions in Super-Kamiokande.
SCIFI sheet making ('98.06)
The fine grain "SCIFI" (SCIntillating FIber) detector will measure
the neutrino flux with very precise information on position and direction
of produced particles. Each fiber is 700 microns in diameter. Many layers
of fiber sheets are arranged between water target containers. This ability
to measure the position and direction accurately is necessary to verify
our understanding of the beam distribution and our physics interaction
models that we use in data analysis.
After production of all sheets, the sheets and target modules will be installed
in the front detector hall just in front of the lead glass detector.
The 1 Kton water Cherenkov tank was installed in the experimental hall.
This is an action photo of lowering the tank into its place for the experiment.
Moving a large steel tank is not as easy as it sounds when it is that large!
This 9 meter structure must be placed with an accuracy of at worst a few
millimeters. The "spider" structure on top is for lifting the
40 ton can; it will be removed soon. The tank looks shiny because it is
wrapped in an insulating layer to make it easier to control the water temperature.
The thin vertical lines that go from the top all the way down are part
of a system of magnetic coils to cancel the magnetic field of the earth.
Soon, this tank will be filled with one thousand tons of very pure water
as several hundred ultra-sensitive light detectors ("photo-multiplier
tubes", or simply "PMTs") are installed. Then it will be
ready to detect neutrinos the same way that the far detector Super-Kamiokande
has been doing for its observation of atmospheric neutrinos. This similarity
is crucial to verifying that all of our detectors are properly understood.